Tumor necrosis factor-{alpha} amplifies adipose-derived chemerin production and bioactivation

Abstract

Due to its escalating prevalence, obesity is becoming a leading cause of morbidity and mortality worldwide. Obesity is a complex health problem accompanied by metabolic abnormalities and low-grade inflammation that increases the risk for developing comorbidities including type 2 diabetes. Recent evidence supports a role for fat (adipose) tissue derived factors, called adipokines, in the development of obesity and obesity-related metabolic pathologies. Chemerin is an adipokine that mediates immune and metabolic effects through the chemokine-like receptor 1 (CMKLR1). Chemerin is secreted as an inactive proform, prochemerin, which subsequently undergoes enzymatic cleavage into multiple chemerin products that differentially activate CMKLR1. Multiple studies have reported elevated total chemerin (a combination of prochemerin and various chemerin products) in obese humans suggesting chemerin involvement in obesity pathophysiology. However, the observational nature of these human studies have restricted them from identifying specific forms of chemerin that are elevated in obesity and the mechanisms that govern them. Herein, I have reported that the levels of both serum total chemerin and chemerin products capable of activating CMKLR1 are elevated in obese mice and in wild type mice following treatment with an obesity-associated inflammatory mediator tumor necrosis factor-? (TNF?). Likewise, cultured adipocytes produced active chemerin under basal conditions and highly active chemerin following TNF? treatment as measured by CMKLR1 activation. The current belief is that prochemerin circulates through blood primed for activation by immune and fibrinolytic enzymes present within injured tissues. My results challenge this theory, identifying adipocytes as cells alone produce and proteolytically activate chemerin. Under basal conditions, a balance between activating serine proteases and deactivating aminopeptidases governed the amount of CMKLR1-activating chemerin formed by adipocytes. Treatment of adipocytes with TNF? elevated the levels of serine proteases elastase and tryptase, which cumulatively shifted the proteolytic balance toward the production of chemerin products that highly activate CMKLR1. Taken together, my results are the first to identify that local TNF? triggers increased adipocyte production of chemerin providing an explanation for the elevated concentrations of chemerin in obese animals and humans. Furthermore, adipocyte processing represents a novel mechanism that likely governs the amount and type of circulating chemerin in obesity.